Heat sensitive recording material using microcapsules containing ultraviolet absorber

ABSTRACT

The present invention provides a heat sensitive recording material comprising a substrate, a recording layer formed over the substrate and containing a colorless or light-colored basic dye and a color acceptor, and a protective layer formed over the recording layer, the recording material being characterized in that microcapsules having an ultraviolet absorber enclosed therein and having substantially no color forming ability are incorporated in the protective layer. 
     Further, the present invention provides microcapsules having an ultraviolet absorber and as required an organic solvent enclosed therein, which have capsule wall film of synthetic resin and mean particle size of 0.1 to 3 μm.

The present invention relates to heat sensitive recording materialswhich are excellent in preservation stability especially in resistanceto light, and microcapsules having an ultraviolet absorber enclosedtherein, which are applicable to various fields.

Heat sensitive recording materials are well known which utilize a colorforming reaction between a colorless or light-colored basic dye and anorganic or inorganic color acceptor to obtain record images by thermallybringing the two chromogenic substances into contact with each other.Such heat sensitive recording materials are relatively inexpensive, areadapted for use with recording devices which are compact and easy tomaintain, and have therefore found wide applications as recording mediafor facsimile systems, various computers, etc.

As one of these applications, heat sensitive recording materials areused in the form of labels, for example, for the POS (point of sales)system. Aside from such labels which are used for food products and arein service usually for a short period of time, growing uses of thissystem involve increased use of heat sensitive recording materials astag sheets which are used as affixed to commodities for a long period oftime, or as handy terminal (portable or pocketable thermal printer)sheets for the collection and delivery of goods which are handledfrequently outdoors. In such use, however, the recording material isoften exposed to indoor light or sunlight for a prolonged period of timeand becomes yellowed over the background area of the material under theinfluence of the light, consequently seriously impairing the image ofthe commercial product. It is therefore strongly desired to provide heatsensitive recording materials which will not yellow over the backgroundarea even when exposed to indoor light or sunlight for a prolongedperiod of time.

In order to improve light resistance of heat sensitive recordingmaterials, it has already been proposed to add a finely dividedultraviolet absorber to the heat sensitive recording layer or protectivelayer. However, the finely divided ultraviolet absorber is low inultraviolet ray absorption efficiency and therefore fails to achieve asufficient effect, while use of an increased amount of the absorberentails another drawback such as background fog or a lower recordingdensity. As a result, fully satisfactory resistance to light stillremains to be obtained presently.

In the case of heat sensitive recording materials for use as tag sheetsor handy terminal sheets, the images recorded thereon must also beresistant to plasticizers, solvents, oils, fats or secum, etc., so thata protective layer is generally provided over the recording layer. Whenthe protective layer contains the finely divided ultraviolet absorber,the absorber dissolves out owing to the influence of a plasticizer, oil,fat or the like, impairing the function of the protective layer andconsequently rendering record images less preservable.

Further, in order to enhance light resistance, it is proposed to coat orapply a pigment which shades ultraviolet ray such as zinc oxide ortitanium oxide, or ultraviolet absorber to various sheets in addition toa heat sensitive recording material, for example, to a pressuresensitive recording sheet, ink-jet recording sheet, image-receivingsheet for thermal dye-transfer recording or like recording media,clay-coated paper for printing or like sheets, fibers, fabrics, etc.

However, when obtaining excellent light resistance by use of finelypulverized ultraviolet ray shading agent or ultraviolet absorber, it isnecessary to use a comparably large amount of such an agent or absorber,which in turn brings high shading effect and deteriorates record images,printed images or color.

An object of the present invention is to provide a heat sensitiverecording material which has a protective layer over a recording layerand which is excellent in light resistance free of yellowing of thebackground area over a very long period of time without entailing areduction in recording density or impaired preservation stability ofrecord images.

An another object of the present invention is to provide microcapsuleswhich have excellent retainability of ultraviolet absorber, difficult tobe ruptured at a usual pressure and are excellent in ultraviolet rayabsorbing efficiency.

The above and other objects of the invention will become apparent fromthe following description.

The present invention provides a heat sensitive recording materialcomprising a substrate, a recording layer formed over the substrate andcontaining a colorless or light-colored basic dye and a color acceptor,and a protective layer formed over the recording layer, the recordingmaterial being characterized in that microcapsules having an ultravioletabsorber enclosed therein and having substantially no color formingability are incorporated in the protective layer.

Further, the present invention provides microcapsules having anultraviolet absorber and as required an organic solvent enclosedtherein, which have capsule wall film of synthetic resin and meanparticle size of 0.1 to 3 μm.

The followings are specific examples of ultraviolet absorbers used inthe present invention.

Phenyl salicylate, p-tert-butylphenyl salicylate, p-octylphenylsalicylate and like salicylic acid type ultraviolet absorbers;

2,4-Dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-octyloxybenzophenone, 2-hydroxy-4-docecyloxybenzophenone,2,2'-dihydroxy-4-methoxybenzophenone,2,2'-dihydroxy-4,4'-dimethoxybenzophenone,2-hydroxy-4-methoxy-5-sulfobenzophenone and like benzophenone typeultraviolet absorbers;

2-Ethylhexyl 2-cyano-3,3-diphenylacrylate, ethyl2-cyano-3,3-diphenylacrylate and like cyanoacrylate type ultravioletabsorbers;

Bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate,bis(2,2,6,6-tetramethyl-4-piperidyl) succinate,bis(1,2,2,6,6-pentamethyl-4-piperidyl)2-(3,5-di-tert-butyl-4-hydroxybenzyl)-2-n-butyl malonate and likehindered amine type ultraviolet absorbers;

2-(2'-Hydroxyphenyl)benzotriazole,2-(2'-hydroxy-5'-methylphenyl)benzotriazole,2-(2'-hydroxy-5'-tert-butylphenyl)benzotriazole,2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole,2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole,2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-chlorobenzotriazole,2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-tert-butylbenzotriazole,2-(2'-hydroxy-3',5'-di-tert-amylphenyl)benzotriazole,2-(2'-hydroxy-3',5'-di-tert-amyphenyl)-5-tert-amylbenzotriazole,2-(2'-hydroxy-3',5'-di-tert-amylphenyl)-5-methoxybenzotriazole,2-[2'-hydroxy-3'-(3",4",5",6"-tetrahydrophthalimido-methyl)-5'-methylphenyl]benzotriazole,2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-tert-butylphenyl)benzotriazole,2-(2'-hydroxy-3'-tert-amyl-5'-phenoxyphenyl)-5-methylbenzotriazole,2-(2'-hydroxy-5'-n-dodecylphenyl)benzotriazole,2-(2'-hydroxy-5'-sec-octyloxyphenyl)-5-phenylbenzotriazole,2-(2'-hydroxy-3'-tert-amyl-5'-phenylphenyl)-5-methoxybenzotriazole, 2-[2'-hydroxy-3',5'-bis(α,α-dimethylbenzyl)phenyl]benzotriazole and likebenzotriazole type ultraviolet absorbers which are solid at ordinarytemperature;

2-(2'-Hydroxy-3'-dodecyl-5'-methylphenyl)-benzotriazole,2-(2'-hydroxy-3'-undecyl-5'-methylphenyl)-benzotriazole,2-(2'-hydroxy-3'-tridecyl-5'-methylphenyl)-benzotriazole,2-(2'-hydroxy-3'-tetradecyl-5'-methylphenyl)-benzotriazole,2-(2'-hydroxy-3'-pentadecyl-5'-methylphenyl)-benzotriazole,2-(2'-hydroxy-3'-hexadecyl-5'-methylphenyl)-benzotriazole,2-[2'-hydroxy-4'-(2"-ethylhexyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(2"-ethylheptyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(2"-ethyloctyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(2"-propyloctyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(2"-propylheptyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(2"-propylhexyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(1"-ethylhexyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(1"-ethylheptyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(1"-ethyloctyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(1"-propyloctyl)oxyphenyl] -benzotriazole,2-[2'-hydroxy-4'-(1"-propylheptyl)oxyphenyl]-benzotriazole,2-[2'-hydroxy-4'-(1"-propylhexyl)oxyphenyl]-benzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-tert-butylphenyl)-5-n-butylbenzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-tert-butylphenyl)-5-tert-pentylbenzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-tert-butylphenyl)-5-n-pentylbenzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-tert-pentylphenyl)-5-tert-butylbenzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-tert-pentylphenyl)-5-n-butylbenzotriazole,2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-sec-butylbenzotriazole,2-(2'-hydroxy-3',5'-di-tert-pentylphenyl)-5-sec-butylbenzotriazole,2-(2'-hydroxy-3'-tert-butyl-5'-tert-pentylphenyl)-5-sec-butylbenzotriazole,2-(2'-hydroxy-3',5'-di-sec-butylphenyl)-5-chlorobenzotriazole,2-(2'-hydroxy-3',5'-di-sec-butylphenyl)-5-methoxybenzotriazole,2-(2'-hydroxy-3',5'-di-sec-butylphenyl)-5-tert-butylbenzotriazole,2-(2'-hydroxy-3',5'-di-sec-butylphenyl)-5-n-butyl-benzotriazole, octyl5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzene-propionate,condensate of methyl3-[3-tert-butyl-5-(2H-benzotriazole-2-yl)-4-hydroxyphenyl]propionate andpolyethylene glycol (molecular weight: about 300) and like benzotriazoletype ultraviolet absorbers which are liquid at ordinary temperature. Ofcourse, the ultraviolet absorber is not limited to thereabove and can beused as required in a mixture of at least two of them. The ordinarytemperature means about 20° C.

Among the above ultraviolet absorbers, preferable is benzotriazole typeabsorber. Among benzotriazole compounds which are solid at ordinarytemperature, preferably used are2-(2'-hydroxy-5'-methylphenyl)benzotriazole,2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole and2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole whichexhibit excellent light-reisitance improving effect.

A benzotriazole compound which is liquid at ordinary temperature is moreadvantageous than a benzotriazole compound which is solid at ordinarytemperature, because the former is not required to be solved in anorganic solvent or dissolved in a small amount thereof for adjusting theviscosity when preparing oil-in-water microcapsules and thus thebenzotriazole compound can be incorporated in microcapsules at a highconcentration which brings particularly improved ultraviolet rayabsorbing ability.

Among benzotriazole compounds which are liquid at ordinary temperature,preferable are 2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole,2-[2'-hydroxy-4'-(2"-ethylhexyl)oxyphenyl]benzotriazole, octyl5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzene-propionateand condensate of methyl3-[3-tert-butyl-5-(2H-benzotriazole-2-yl)-4-hydroxyphenyl]propionate andpolyethylene glycol (molecular weight: about 300). Among these,particularly preferable are2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole,2-[2'-hydroxy-4'-(2"-ethylhexyl)oxyphenyl]benzotriazole and octyl5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzene-propionatewhich easily give microcapsules having excellent retainability, and donot afford adverse effect on preservation stability of record imageswhen used in preparing, for example, a heat sensitive recordingmaterial.

Although the amount of ultraviolet absorber to be used in a heatsensitive recording material is not limited specifically, it isdesirable to adjust the amount preferably to 10 to 500 parts by weight,more preferably to 20 to 250 parts by weight, per 100 parts by weight ofthe basic dye present in the heat sensitive recording layer.

The important feature of the present invention is that microcapsuleshaving the above-described ultraviolet absorber enclosed therein andhaving substantially no color forming ability are incorporated in theprotective layer to be formed over the heat sensitive recording layer."Microcapsules having substantially no color forming ability" means thatthose not containing a basic dye or a color acceptor, the former forms acolor in contact with a color acceptor (acidic substance) and the latterforms a color in contact with a basic dye. Although various heat- orpressure-sensitive recording materials have been proposed which comprisemicrocapsules having enclosed therein an ultraviolet absorber along witha colorless or light-colored basic dye, these microcapsules contain thebasic dye and therefore become colored with an increase in the amount oflight to which they are exposed, further failing to exhibit asatisfactory effect to give improved light resistance over a longperiod.

The microcapsules for use in the present invention can be prepared byvarious known methods. They are prepared generally by emulsifying anddispersing the core material (oily liquid) comprising an ultravioletabsorber and, as required, an organic solvent in an aqueous medium, andforming a wall film of high-molecular-weight substance around theresulting oily droplets.

Examples of useful high-molecular-weight substances for forming the wallfilm of microcapsules are polyurethane resin, polyurea resin, polyamideresin, polyester resin, polycarbonate resin, aminoaldehyde resin,melamine resin, polystyrene resin, styrene-acrylate copolymer resin,styrene-methacrylate copolymer resin, gelatin, polyvinyl alcohol, etc.Especially, microcapsules having a wall film of a synthetic resin,particularly polyurea resin, polyurethane.polyurea resin andaminoaldehyde resin among other resins have excellent retainability ofan ultraviolet absorber and high heat resistance and accordingly exhibitthe outstanding additional effect to serve the function of a pigmentwhich is to be incorporated in the protective layer for preventingsticking to the thermal head. Moreover, microcapsules having a wall filmof polyurea resin or polyurethane.polyurea resin are lower in refractiveindex than microcapsules with wall films of other materials and usualpigments, are spherical in shape and are therefore usable favorablybecause even if present in a large quantity in the protective layer,they are unlikely to reduce the density of record images (so-calledwhitening) owing to irregular reflection of light. Further, polyurearesin and polyurethane.polyurea resin are more elastic thanaminoaldehyde resin and therefore polyurea resin andpolyurethane.polyurea resin are preferably used as a wall film formicrocapsules which are used under a condition of high pressure. On theother hand, microcapsules having a wall film made from aminoaldehyderesin have a merit that the wall film can be controlled in thicknesswithout depending on particle size of emulsion because the microcapsulescan be prepared by adding a wall-forming material after emulsificationof a core material.

Microcapsules having a wall film of polyurea resin orpolyurethane.polyurea resin are disclosed for example inJP-B-19574/1963, 446/1967, 771/1967 and U.S. Pat. No. 3,796,669 and areprepared with use of a polyisocyanate, a combination of a polyisocyanateand a polyol reactive therewith, an adduct of polyisocyanate and polyol,or like capsule wall-forming material, by admixing the wall-formingmaterial with the core material to be encapsulated, emulsifying anddispersing the mixture in an aqueous medium containing polyvinyl alcoholor like protective colloid, and raising the temperature of thedispersion to cause a polymer forming reaction at the interface betweenthe oily droplets and the aqueous medium.

Examples of polyisocyanate compounds are m-phenylene diisocyanate,p-phenylene diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylenediisocyanate, naphthalene-1,4diisocyanate,diphenylmethane-4,4'-diisocyanate,3,3'-dimethyldiphenylmethane-4,4'-diisocyanate,xylylene-1,4-diisocyanate, 4,4'-diphenylpropane diisocyanate,trimethylene diisocyanate, hexamethylene diisocyanate,propylene-1,2-diisocyanate, butylene-1,2-diisocyanate,cyclohexylene-1,2-diisocyanate, cyclohexylene-1,4-diisocyanate and likediisocyanates, 4,4',4"-triphenylmethane triisocyanate,toluene-2,4,6-triisocyanate and like triisocyanates,4,4'-dimethyldiphenylmethane-2,2',5,5'-tetraisocyanate and liketetraisocyanates, adduct of hexamethylene diisocyanate andtrimethylolpropane, adduct of 2,4-tolylene diisocyanate andtrimethylolpropane, adduct of xylylene diisocyanate andtrimethylolpropane, adduct of tolylene diisocyanate and hexanetriol andlike isocyanate prepolymers.

Examples of polyols are ethylene glycol, 1,3-propanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol,1,8-octanediol, propylene glycol, 2,3-dihydroxybutane,1,2-dihydroxybutane, 1,3-dihydroxybutane,2,2-dimethyl-1,3-propanonediol, 2,4-pentanediol, 2,5-hexanediol,3-methyl-1,5-pentanediol, 1,4-cyclohexanedimethanol,dihydroxycyclohexane, diethyleneglycol, 1,2,6-trihydroxyhexane,phenylethyleneglycol, 1,1,1-trimethylolpropane, hexanetriol,pentaerythritol, glycerin and like aliphatic polyols,1,4-di(2-hydroxyethoxy)benzene, 1,3-di(2-hydroxyethoxy)benzene and likecondensates of aromatic polyalcohol and alkylene oxide, p-xylyleneglycol, m-xylylene glycol, α, α'-dihydroxy-p-diisopropylbenzene,4,4'-dihydroxydiphenylmethane,2-(p,p'-dihydroxydiphenylmethyl)benzylalcohol,4,4'-isopropylidenediphenol, 4,4'-dihydroxydiphenylsulfone,4,4'-dihydroxydiphenylsulfide, adduct of 4,4'-isopropylidenediphenol andethylene oxide, adduct of 4,4'-isopropylidenediphenol and propyleneoxide.

The polyisocyanate compound and polyol are not limited to thereabove andat least two of them can be used as required.

Among the polyisocyanate compounds, or combinations or adducts of thepolyisocyanate and polyol, preferable are those having at least threeisocyanate groups in the molecule. More preferable is an adduct oftrimethylolpropane or like polyol compound and xylylene diisocyanatewhich is generally highly viscous and requires a solvent for dilutionbut the wall film thereof hardly yellows by the exposure of light.Further, more preferable is hexamethylene diisocyanate derivative havinga viscosity of up to 1000 cps, preferably up to 800 cps at 25° C. byBrookfield viscometer which does not necessarily require a solvent fordilution and the wall film thereof hardly yellows by the exposure oflight. Examples of these hexamethylene diisocyanate derivatives areisocyanurate type hexamethylene diisocyanate (cyclic trimer ofhexamethylene diisocyanate), biuret type hexamethylene diisocyanate(trimer of hexamethylene diisocyanate) and isocyanate compoundscontaining a mixture of trimer, pentamer and higher oligomers ofhexamethylene diisocyanate. Particularly preferable are isocyanatecompounds containing as a main component isocyanurate type trimer ofhexamethylene diisocyanate which are excellent in solubility withultraviolet absorber.

Microcapsules having a wall film made of aminoaldehyde resin aredisclosed, for example, in JP-B-9168/1961, U.S. Pat. No. 4,001,140 andU.S. Pat. No. 4,100,103. These microcapsules are generally prepared byin-situ polymerization wherein a core material is emulsified in anaqueous medium containing an emulsifier, thereafter a wall-formingmaterial is added thereto and accumulated on the surface of the corematerial by adjusting conditions of heat and pH.

As a wall-forming material, it is used a combination of an amine andaldehyde, or a precondensate of these two compounds. Examples of aminesare urea, thiourea, alkyl urea, ethylene urea, acetoguanamine,benzoguanamine, melamine, guanidine, biuret, cyanamide andhexamethylenetetramine. Examples of aldehydes are formaldehyde,acetaldehyde, paraformaldehyde, glutaraldehyde, glyoxal and furfural.

In the present microcapsules can be contained as required an organicsolvent together with a ultraviolet absorber. The organic solvent is notparticularly limited and various hydrophobic solvents can be used whichare used in a field of pressure sensitive manifold papers. Examples oforganic solvents are tricresyl phosphate, octyldiphenyl phosphate andlike phosphates, dibutyl phthalate, dioctyl phthalate and likephthalates, butyl oleate and like carboxylates, various fatty acidamides, diethylene glycol dibenzoate, monoisopropylnaphthalene,diisopropylnaphthalene and like alkylated naphthalenes,1-methyl-1-phenyl-1-tolylmethane, 1-methyl-1-phenyl-1-xylylmethane,1-phenyl-1-tolylmethane and like alkylated benzenes, isopropylbiphenyland like alkylated biphenyls, trimethylolpropane triacrylate and likeacrylates, ester of polyol and unsaturated carboxylic acid, chlorinatedparaffin and kerosene. These solvents can be used as required in amixture of at least two of them. Among these hydrophobic media having ahigh boiling point, tricresyl phosphate and 1-phenyl-1-tolylmethane aredesirable since they exhibit high solubility in connection with theultraviolet absorber to be used in the present invention. Generally, thelower the viscosity of the core material, the smaller is the particlesize resulting from emulsification and the narrower is the particle sizedistribution, so that a solvent having a low boiling point is conjointlyusable to lower the viscosity of the core material. Examples of suchsolvents having a low boiling point are ethyl acetate, butyl acetate,methylene chloride, etc.

The amount of organic solvent to be used should be suitably adjustedaccording to the kind and amount of ultraviolet absorber to be used andthe kind of organic solvent and is not limited specifically. For examplein case of using an ultraviolet absorber which is liquid at ordinarytemperature, an organic solvent is not necessarily used. However, incase of using an ultraviolet absorber which is solid at ordinarytemperature, since it is desired that the ultraviolet absorber be in afully dissolved state in the microcapsules, the amount of organicsolvent, for example in case of microcapsules of polyurea resin orpolyurethane.polyurea resin, is adjusted preferably to usually 10 to 60wt. %, more preferably to 20 to 60 wt. %, based on the combined amountof organic solvent, ultraviolet absorber and wall-forming material.Further, in case of microcapsules of aminoaldehyde resin, the amount oforganic solvent is adjusted to usually 50 to 2000% by weight, preferably100 to 1000% by weight of ultraviolet absorber.

While the amount of capsule wall-forming material to be used is notlimited specifically either, preservation for a long period of time islikely to permit the organic solvent in the microcapsules to ooze out todecrease contemplated effects or give adverse effects to a heatsensitive recording material and other materials having microcapsulesused, so that it is desired to use a larger amount of wall-formingmaterial than is the case with usual microcapsules used in a pressuresensitive recording material, etc. Thus, for example in case of usingmicrocapsules of polyurea resin or polyurethane.polyurea resin, thewall-forming material is used preferably in an amount of 20 to 70 wt. %,more preferably 25 to 60 wt. %, based on the combined amount of thethree components, i.e., the organic solvent which is used as required,ultraviolet absorber and wall-forming material. In case of usingmicrocapsules of aminoaldehyde resin, the wall-forming material is usedusually in an amount of 30 to 300% by weight, preferably 35 to 200% byweight of the core material containing as main components ultravioletabsorber and as required organic solvent.

The amount of ultraviolet absorber to be used for encapsulation shouldbe determined suitably according to the kind of ultraviolet absorber andorganic solvent to be used, etc. and is not limited specifically. Toobtain a remarkable effect, however, it is desirable to use the absorberin an amount of 3 to 80 wt. %, preferably 3 to 75 wt. %, based on thecombined amount of the three components, i.e., the organic solvent whichis used as required, ultraviolet absorber and wall-forming material.

Emulsifiers (protective colloid agents) useful for microencapsulationare various anionic, nonionic, cationic or amphoteric water-soluble highpolymer compounds and the like. These emulsifiers are used as selecteddepending on the kind of the wall-forming material.

Useful anionic high polymer compounds are natural or synthetic highpolymer compounds having --COO⁻, --SO₃ ⁻, --OPO₃ ²⁻ or like group. Theyare, for example, natural high polymer compounds such as gum arabic,carrageenan, sodium alginate, pectic acid, tragacanth, armond gum andagar, semisynthetic high polymer compounds such as carboxymethylcellulose, cellulose sulfate, methyl cellulose sulfate,carboxymethylated starch, phosphorylated starch and ligninsulfonic acid,and synthetic high polymer compounds such as copolymers of maleicanhydride (including one as hydrolyzed), polymers and copolymers ofacrylic acid, methacrylic acid or crotonic acid, polymers and copolymersof vinylbenzenesulfonic acid or 2-acrylamido-2-methylpropanesulfonicacid, partial amide derivatives or partially esterified products of suchpolymers or copolymers, acetoacetyl-modified polyvinyl alcohol,carboxyl-modified polyvinyl alcohol, sulfonic acid-modified polyvinylalcohol and phosphoric acid-modified polyvinyl alcohol.

More specifically, examples of useful copolymers of maleic anhydride(inclusive of product of hydrolysis thereof) are methyl vinylether-maleic anhydride copolymer, ethylene-maleic anhydride copolymer,styrene-maleic anhydride copolymer, α-methylstyrene-maleic anhydridecopolymer, vinyl acetate-maleic anhydride copolymer,methacrylamide-maleic anhydride copolymer, isobutylene-maleic anhydridecopolymer and the like. Examples of useful acrylic acid copolymers,methacrylic acid copolymers or crotonic acid copolymers are methylacrylate-acrylic acid copolymer (hereinafter "copolymer" is omitted),ethyl acrylate-acrylic acid, methyl acrylate-methacrylic acid, methylmethacrylate-acrylic acid, methyl methacrylate-methacrylic acid, methylacrylate-acrylamide-acrylic acid, acrylonitrile-acrylic acid,acrylonitrile-methacrylic acid, hydroxyethyl acrylate-acrylic acid,hydroxyethyl methacrylate-methacrylic acid, vinyl acetate-acrylic acid,vinyl acetate-methacrylic acid, acrylamide-acrylic acid,acrylamide-methacrylic acid, methacrylamide-acrylic acid,methacrylamide-methacrylic acid, vinyl acetate-crotonic acid and likecopolymers. Examples of useful vinylbenzenesulfonic acid or2-acrylamide-2-methylpropanesulfonic acid copolymers are methylacrylate-vinylbenzenesulfonic acid (or salt thereof) copolymer, vinylacetate-vinylbenzenesulfonic acid copolymer,acrylamide-vinylbenzenesulfonic acid copolymer,acryloylmorpholine-vinylbenzenesulfonic acid copolymer,vinylpyrrolidone-vinylbenzenesulfonic acid copolymer,vinylpyrrolidone-2-methylpropanesulfonic acid copolymer and the like.

Examples of useful nonionic high polymer compounds are semisynthetichigh poplymer compounds having an --OH group, such as hydroxyethylcellulose, methyl cellulose, pullulan, soluble starch and oxidizedstarch, synthetic high polymer compounds such as polyvinyl alcohol andsilicon-modified polyvinyl alcohol. Examples of useful cationic highpolymer compounds are cation-modified polyvinyl alcohol and the like.Examples of useful amphoteric high polymer compounds are gelatin and thelike.

Among the above emulsifiers, a completely or partially saponifiedcompound of a modified polyvinyl alcohol such as acetoacetyl-modifiedpolyvinyl alcohol, carboxyl-modified polyvinyl alcohol andsilicon-modified polyvinyl alcohol is preferably used when the resultingmicrocapsule dispersion is applied, for example, to a protective layerof a heat sensitive recording material which requires excellent waterresistance, because the above modified polyvinyl alcohol affordsexcellent water resistance easily with conjoint use of a hardener.

Although the amount of emulsifier to be used is not limitedspecifically, for example in case of using microcapsules of polyurearesin or polyurethane.polyurea resin, it is generally adjusted to 1 to50 wt. %, preferably to 3 to 30 wt. %, based on the combined amount ofthe three components, i.e., the wall-forming material, ultravioletabsorber and organic solvent. In case of using microcapsules ofaminoaldehyde resin, the emulsifier is used usually in an amount of 1 to20% by weight, preferably 3 to 10% by weight, based on the combinedamount of ultraviolet absorber and organic solvent.

In addition to the ultraviolet absorber, antioxidants, oil-solublefluorescent dyes, releasing agents, etc. can be incorporated into themicrocapsules of the invention when so desired.

Also usable in combination with the foregoing components forencapsulation of microcapsules of polyurea resin andpolyurethane.polyurea resin are tin compounds, polyamide compounds,epoxy compounds, polyamine compounds, etc. as reaction accelerators. Inthe case where polyamine compounds are used, it is desirable to usealiphatic polyamine compounds in view of resistance to light.

When the ultraviolet ray absorbing efficiency, compressive strength ofmicrocapsule, the quality of images of heat sensitive recording materialor the like to be recorded, etc. are considered, it is desirable thatthe microcapsules to be used in the present invention be adjusted to 0.1to 3 μm, preferably 0.3 to 2.5 μm, in mean particle size.

The amount of microcapsules when incorporated into the protective layerof heat sensitive recording material should be suitably determined inaccordance with the kind of ultraviolet absorber, the concentrationthereof in the capsules and the desired quality of recording material tobe obtained. It is generally desirable to adjust the amount to 5 to 80wt. %, preferably 20 to 70 wt. %, based on the total solids content ofthe protective layer.

The amount of ultraviolet absorber to be used in the protective layershould be adjusted depending on the kind of ultraviolet absorberenclosed in microcapsules and the desired quality of heat sensitiverecording material. To improve remarkably light resistance of heatsensitive recording material, particularly to prevent yellowing ofbackground area, it is desirable to apply microcapsules in an amount of,as the amount of ultraviolet absorber, usually 0.1 to 3.0 g/m²,preferably 0.2 to 2.0 g/m².

Further, the amount of microcapsules used for recording media other thanheat sensitive recording material, coated papers for printing or likesheets, or other fibers should be determined suitably according to thedegree of desired light resistance.

The present microcapsule thus obtained is not only excellent inultraviolet ray absorption efficiency but also excellent inretainability of ultraviolet absorber and compressive strength.Accordingly, the present microcapsule is usable for improving lightresistance of various materials such as a protective layer of heatsensitive recording material or like recording materials and a vanishfor top coating of various printed matters.

In case of using microcapsules in the form of powder, the aqueousmicrocapsule dispersion is made into a powder of microcapsules by dryingthe dispersion as prepared, as concentrated by filtration or the like,or as treated by the method disclosed in U.S. Pat. No. 4,601,863. Thedispersion is dried by air drying, flow drying, air-stream drying, spraydrying, vacuum drying, freeze drying, infrared drying, high-frequencydrying, ultrasonic drying or like method. In case of using microcapsulesin the form of a dispersion in a non-aqueous medium, such a dispersioncan be prepared by admixing a non-aqueous medium with the aqueousmicrocapsule dispersion after filtration and removing the aqueous mediumfrom the dispersion. Alternately, the microcapsule powder obtained aboveis dispersed in a non-aqueous medium to obtain a non-aqueousmicrocapsule dispersion.

The following is a detailed description about an application of thepresent microcapsule to a protective layer of a heat sensitive recordingmaterial.

In the present heat sensitive recording material, various knowncolorless or light-colored basic dyes can be used as a basic dyeincorporated in the heat sensitive recording layer. Examples thereof areas follows.

Blue-color dyes, e.g.,3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide,3-(4-diethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)-6-dimethylaminophthalide,3-diethylamino-7-dibenzylamino-benzo[a]fluoran, etc.

Green-color dyes, e.g.,3-(N-ethyl-N-p-tolyl)amino-7-methylanilinofluoran,3-diethylamino-7-anilinofluoran, 3-diethylamino-7-dibenzylaminofluran,etc.

Red-color dyes, e.g., 3,6-bis(diethylamino)fluoran-τ-anilinolactam,3-cyclohexylamino-6-chlorofluoran,3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-7-chlorofluoran,rhodamine (o-chloroanilino) lactam, rhodamine (p-chloroanilino)lactam,3-diethylamino-7,8-benzofluoran,3-(N-ethyl-p-toluidino)-7-methylfluoran,3-diethylamino-6,8-dimethylfluoran, etc.

Black-color dyes, e.g., 3-(N-ethyl-N-isoamyl)amino-6-methyl-7-anilinofluoran, 3- (N-methyl-N-cyclohexyl)amino-6-methyl-7-anilinofluoran,3-diethylamino-6-methyl-7-anilinofluoran, 3-di (n-butyl)amino-6-methyl-7-anilinofluoran, 3-di (n-pentyl)amino-6-methyl-7-anilinofluoran,3-diethylamino-7-(o-chlorophenylamino)fluoran,3-di(n-butyl)amino-7-(o-chlorophenylamino)fluoran,3-diethylamino-7-(o-fluorophenylamino)fluoran,3-di(n-butyl)amino-7-(o-fluorophenylamino)-fluoran,3-diethylamino-7-(m-trifluoromethylphenylamino)-fluoran,3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran,3-(N-ethyl-p-toluidino)-6-methyl-7-(p-toluidino)fluoran,3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran,3-diethylamino-6-chloro-7-anilinofluoran,3-(N-methyl-N-n-propylamino)-6-methyl-7-anilinofluoran,3-dimethylamino-6-methyl-7-anilinofluoran,3-di(n-butyl)amino-6-methyl-7-m-toluidinofluoran,3-(N-n-hexyl-N-ethyl)amino-6-methyl-7-anilinofluoran,3-(N-ethyl-N-isobutyl)amino-6-methyl-7-anilinofluoran,3-diethylamino-6-methyl-7-p-ethoxyanilinofluoran,3-pyrrolidino-6-methyl-7-anilinofluoran,3-piperidino-6-methyl-7-anilinofluoran,2,2-bis{4-[6'-(N-cyclohexyl-N-methylamino)-3'-methylspiro[phthalide-3,9'-xanthene-2'-ylamino]phenyl}propane,etc.

Dyes having absorption wave in near infrared region, e.g.,3,3-bis[1-(4-methoxyphenyl)-1-(4-dimethylaminophenyl)-ethylene-2-yl]-4,5,6,7-tetrachlorophthalide,3,3-bis[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide,3,3-bis[1,1-bis(4-pyrrolidinophenyl)-ethylene-2-yl]-4,5,6,7-tetrabromophthalide,3-p-(p-dimethylaminoanilino)anilino-6-methyl-7-chlorofluoran,3-p-(p-chloroanilino)anilino-6-methyl-7-chlorofluoran,3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylamino)-phthalide,etc. Of course, the basic dye is not limited to thereabove and can beused as required in a mixture of at least two of them.

Various known color acceptors are usable in combination with the abovebasic dye. Examples thereof are as follows.

Inorganic color acceptors:

Activated clay, attapulgite, colloidal silica, aluminum silicate, etc.

Organic color acceptors (organic acidic substances):

Phenolic compounds, e.g., 4,4'-isopropylidenediphenol,1,1-bis(4-hydroxyphenyl)cyclohexane,2,2-bis(4-hydroxyphenyl)-4-methylpentane, 4,4'-dihydroxydiphenylsulfide,hydroquinone monobenzyl ether, benzyl 4-hydroxybenzoate,4,4'-dihydroxydiphenylsulfone, 2,4'-dihydroxydiphenylsulfone,4-hydroxy-4'-isopropoxydiphenylsulfone,bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenylsulfone,4-hydroxyphenyl-4'-benzyloxyphenylsulfone,3,4-dihydroxyphenyl-4'-methylphenylsulfone,bis(4-hydroxyphenylthioethoxy)methane,1,5-di(4-hydroxyphenylthio)-3-oxapentane, butylbis(p-hydroxyphenyl)acetate, methyl bis(p-hydroxyphenyl)acetate,1,1-bis(4-hydroxyphenyl)-1-phenylethane,1,4-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene,1,3-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene,di(4-hydroxy-3-methylphenyl)-sulfide, 2,2'-thiobis(3-tert-octylphenol),2,2'-thiobis(4-tert-octylphenol), etc.

Thiourea compounds, e.g., N,N'-di-m-chlorophenylthiourea, etc.

Aromatic carboxylic acids, e.g., p-chlorobenzoic acid,4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid,4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid,5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid., etc.

Salt of the aromatic carboxylic acid with a polyvalent metal such aszinc, magnesium, aluminum, calcium, titanium, manganese, tin and nickel.

Metal complex, e.g., complex of antipyrine with zinc thiocyanate.

Composite zinc salt of terephthalaldehydic acid with other aromaticcarboxylic acid.

With the heat sensitive recording materials of the invention, theproportions of basic dye and color acceptor are not particularly limitedbut can be determined suitably according to the kinds of basic dye andcolor acceptor. For example, usually 1 to 50 parts by weight, preferably2 to 10 parts by weight, of the color acceptor is used per part byweight of the basic dye.

For preparing the coating composition for the heat sensitive recordinglayer, the dye and the color acceptor are dispersed, together orindividually, into water serving as a dispersion medium, using stirringand pulverizing means such as a ball mill, attritor or sand mill.

In the coating composition, a binder can be conjointly used. Examples ofuseful binders are starches, hydroxyethyl cellulose, methyl cellulose,carboxymethyl cellulose, gelatin, casein, gum arabic, polyvinyl alcohol,carboxyl-modified polyvinyl alcohol, acetoacetyl-modified polyvinylalcohol, silicon-modified polyvinyl alcohol, diisobutylene-maleicanhydride copolymer salt, styrene-maleic anhydride copolymer salt,ethylene-acrylic acid copolymer salt, styrene-acrylic acid copolymersalt, styrene-butadiene copolymer emulsion, urea resin, melamine resin,amide resin, polyurethane resin, etc. At least one of these binders isadded to the recording layer in an amount of 5 to 30% by weight based onthe total solids of the layer.

Various other auxiliary agents can be further added to the coatingcomposition. Examples of useful agents are dispersants such as sodiumdioctylsulfosuccinate, sodium dodecylbenzenesulfonate, sodium salt oflauryl alcohol sulfuric acid ester, fatty acid metal salts, etc., waxessuch as zinc stearate, calcium stearate, polyethylene wax, carnauba wax,paraffin wax and ester wax, etc, defoaming agents, fluorescent dyes,coloring dyes, etc. Further, in order to enhance light resistance moreeffectively, it is possible to add microcapsules containing ultravioletabsorber or finely pulverized ultraviolet absorber to the coatingcomposition for the recording layer.

Further, to the coating composition may be added a pigment. Examples ofuseful pigments are kaolin, clay, calcium carbonate, calcined clay,calcined kaolin, titanium dioxide, kieselguhr, finely divided anhydroussilica, activated clay or like inorganic pigment, styrene microballs,nylon powder, polyethylene powder, urea-formalin resin filler, particlesof raw starch or like organic pigment.

Further, a sensitizer can be used as required. Examples of usefulsensitizers are stearic acid amide, methoxycarbonyl-N-stearic acidbenzamide, N-benzoyl stearic acid amide, N-eicosanic acid amide,ethylenebis stearic acid amide, behenic acid amide, methylenebis stearicacid amide, N-methylol stearic acid amide, dibenzyl terephthalate,dimethyl terephthalate, dioctyl terephthalate, benzylp-benzyloxybenzoate, phenyl 1-hydroxy-2-naphthoate, 2-naphthyl benzylether, m-terphenyl, dibenzyl oxalate, di-p-methylbenzyl oxalate,di-p-chlorobenzyl oxalate, p-benzylbiphenyl, p-tolyl biphenyl ether,di(p-methoxyphenoxyethyl) ether, 1,2-di(3-methylphenoxy)ethane,1,2-di(4-methylphenoxy)ethane, 1,2-di(4-methoxyphenoxy)ethane,1,2-di(4-chlorophenoxy)ethane, 1,2-diphenoxyethane,1-(4-methoxyphenoxy)-2-(3-methylphenoxy)-ethane, p-methylthiophenylbenzyl ether, 1,4-di (phenylthio)-butane, p-acetotoluidide,p-acetophenetidide, N-acetoacetyl-p-toluidine,di(β-biphenylethoxy)benzene, p-di(vinyloxyethoxy)-benzene,1-isopropylphenyl-2-phenylethane, etc.

It is desired that the amount of sensitizer to be used be adjustedgenerally within the range of up to about 4 parts by weight per part byweight of the color acceptor although not limited specifically.

It is also possible to add a preservability improving agent for recordimages as required. Examples thereof are as follows.

Hindered phenol compounds, e.g.,

2,2'-methylenebis(4-methyl-6-tert-butylphenol),

2,2'-methylenebis(4-ethyl-6-tert-butylphenol),

2,2'-methylenebis(4,6-di-tert-butylphenol),

2,2'-ethylenebis(4-methyl-6-tert-butylphenol),

2,2'-ethylidenebis(4,6-di-tert-butylphenol),

2,2'-ethylidenebis(4-methyl-6-tert-butylphenol),

2,2'-ethylidenebis(4-ethyl-6-tert-butylphenol),

2,2'-(2,2-propylidene)bis(4,6-di-tert-butylphenol),

2,2'-methylenebis(4-methoxy-6-tert-butylphenol),

2,2'-methylenebis(6-tert-butylphenol),

4,4'-thiobis(3-methyl-6-tert-butylphenol),

4,4'-thiobis(2-methyl-6-tert-butylphenol),

4,4'-thiobis(5-methyl-6-tert-butylphenol),

4,4'-thiobis(2-chloro-6-tert-butylphenol),

4,4'-thiobis(2-methoxy-6-tert-butylphenol),

4,4'-thiobis(2-ethyl-6-tert-butylphenol),

4,4'-butylidenebis(6-tert-butyl-m-cresol),

1-[α-methyl-α-(4'-hydroxyphenyl)ethyl]-4-[α',α'-bis(4"-hydroxyphenyl)ethyl]benzene,

1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane,

1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,

4,4'-thiobis(3-methylphenol),

4,4'-dihydroxy-3,3',5,5'-tetrabromodiphenylsulfone,

4,4'-dihydroxy-3,3',5,5'-tetramethyldiphenylsulfone,

2,2-bis(4-hydroxy-3,5-dibromophenyl)propane,

2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane,

2,2-bis(4-hydroxy-3,5-dimethylphenyl)propane, etc.

Epoxy compounds, e.g., 1,4-diglycidyloxybenzene,4,4'-diglycidyloxydiphenylsulfone,4-benzyloxy-4'-(2-methylglycidyloxy)diphenylsulfone, diglycidylterephthalate, cresol novolak type epoxy resin, phenol novolak typeepoxy resin, bisphenol A type epoxy resin, etc.

N,N'-Di-2-naphthyl-p-phenylenediamine, sodium or polyvalent metal saltof 2,2'-methylenebis(4,6-di-tert-butylphenyl)phosphate,bis(4-ethyleneiminocarbonylaminophenyl)methane, etc.

Among the above preservability improving agents preferable are2,2'-methylenebis(4-methyl-6-tert-butylphenol),2,2'-methylenebis(4-ethyl-6-tert-butylphenol),2,2'-ethylidenebis(4,6-di-tert-butylphenol),4,4'-thiobis(2-methyl-6-tert-butylphenol) and4,4'-thiobis(3-methyl-6-tert-butylphenol) which improve remarkably lightresistance of record images, therefore it is preferable to use when sodesired. The preservability improving agent is used in an amount of 1 to500 parts by weight, preferably 10 to 200 parts by weight per 100 partsby weight of the basic dye.

The present heat sensitive recording material has as described above aprotective layer containing microcapsules having incorporatedultraviolet absorber therein. The protective layer contains as maincomponents microcapsules and a binder. The binder is composed of awater-soluble or water-dispersible high molecular weight compound.

Examples of useful binders are starches, hydroxyethyl cellulose, methylcellulose, carboxymethyl cellulose, gelatin, casein, gum arabic,polyvinyl alcohol, carboxyl-modified polyvinyl alcohol,acetoacetyl-modified polyvinyl alcohol, silicon-modified polyvinylalcohol, diisobutylene-maleic anhydride copolymer salt, styrene-maleicanhydride copolymer salt, ethylene-acrylic acid copolymer salt,styrene-acrylic acid copolymer salt, styrene-butadiene copolymeremulsion, urea resin, melamine resin, amide resin, polyurethane resin,etc. Among the above particularly preferable are acetoacetyl-modifiedpolyvinyl alcohol, carboxyl-modified polyvinyl alcohol andsilicon-modified polyvinyl alcohol which easily afford a film havingexcellent water resistance.

In order to improve amenability to printing and prevent sticking,pigments can be incorporated into the protective layer. Examples ofuseful pigments are inorganic pigments such as calcium carbonate, zincoxide, aluminum oxide, titanium dioxide, silicon dioxide, aluminumhydroxide, barium sulfate, zinc sulfate, talc, kaolin, clay, calcinedkaolin and colloidal silica, and organic pigments such as styrenemicroballs, nylon powder, polyethylene powder, urea-formalin resinfiller and raw starch particles. Generally, such a pigment is usedpreferably in an adjusted amount of about 5 to about 300 parts by weightper 100 parts by weight of the binder.

The method of preparing the coating composition for the protective layeris not particularly limited and the above specific microcapsule(dispersion), binder and as required pigment are dispersed into waterserving as a dispersion medium.

Further when required, various auxiliary agents can be incorporated intothe coating composition for forming the protective layer. Examples ofsuitable agents are lubricants such as zinc stearate, calcium stearate,polyethylene wax, carnauba wax, paraffin wax and ester wax, surfactants(dispersant, wetting agent) such as sodium dioctylsulfosuccinate,defoaming agents, water-soluble polyvalent metal salts such as potassiumalum and aluminum acetate, etc. Also usable are curing agents, such asglyoxal, boric acid, dialdehyde starch and epoxy compounds, for givingimproved water-resistance to the layer.

The heat sensitive recording layer or protective layer can be formed bya method which is not limited specifically, for example, by applying acoating composition for the heat sensitive recording layer to thesubstrate and by applying a coating composition for the protective layerto the recording layer, by a suitable method such as air knife coating,varibar blade coating, pure blade coating, rod blade coating,short-dwell coating, curtain coating or die coating, and drying theresulting respective coating. The substrate is suitably selected frompaper, plastics film, synthetic paper, non-woven fabric, metal-depositedmaterial and the like. The amount of coating composition for therecording layer is adjusted usually to the range of 2 to 12 g/m²,preferably about 3 to about 10 g/m², in dry weight.

The amount of coating composition for the protective layer is adjustedusually to the range of 0.1 to 20 g/m², preferably about 0.5 to about 10g/m², in dry weight.

Further, it is possible to form as required a protective layercontaining a binder and optionally a pigment on the rear surface of theheat sensitive recording material to give improved preservability ofrecord images. Incidentally, various techniques already known in the artof preparing heat sensitive recording materials can be additionallyresorted to. For example, an undercoat layer can be formed on thesubstrate, or the recording material formed with each layer can betreated by supercalendering to smooth the surface. Further, it ispossible to form an adhesive layer on the rear surface of the recordingmaterial to obtain an adhesive label, to form a magnetic recordinglayer, pigment-coated layer for printing or thermal dye-transferrecording layer on the rear surface of the recording material.

In the present heat sensitive recording material, it is desirable toprovide the protective layer surface with smoothness treatment whichimproves the color forming sensitivity and the quality of record images.It is preferable to conduct the treatment to give the surface BekkSmoothness (JIS P 8119) of at least 300 seconds, preferably at least 500seconds. As a device for smoothness treatment is used a supercalender orglosscalender using a combination of an elastic roll and metal roll,etc. In order to prevent rupture of microcapsules incorporated in theprotective layer, it is desirable to use an elastic roll having Shore Dhardness defined in ASTM D-2240 of 60° to 90°, preferably 70° to 90°.The nip pressure at the treatment should be determined suitablyaccording to the hardness of elastic roll and desired smoothness, and isadjusted preferably to 30 to 300 kg/cm.

The elastic layer of the elastic roll is made from natural rubber,styrene rubber, nitrile rubber, chloroprene rubber, chlorosulfonatedethylene rubber, butyl rubber, polysulfide rubber, silicon rubber,fluorinated rubber, urethane rubber and like rubbers, various plasticsresins, cotton, paper, wool, polyethylene terephthalate, nylon, or amixture of these materials. Examples of metal rolls are chilled roll,alloy chilled roll, steel roll, metal roll having hard chromium-platedsurface, etc.

The present invention will be described in greater detail with referenceto the following examples, to which the invention is of course notlimited.

The parts and percentages in these examples are all by weight unlessotherwise specified.

EXAMPLE 1 (1) Composition (A)

To a vessel equipped with a heater was added 60 parts of 8% aqueoussolution of polyvinyl alcohol (brand name: PVA-217, product of KurarayCo., Ltd.) to obtain an aqueous medium for microencapsulation.

Separately, in 12 parts of tricresyl phosphate were disolved 2 parts of2-(2'-hydroxy-5'-methylphenyl)-benzotriazole and 18 parts of an adductof xylylenediisocyanate and trimethylolpropane (3:1) (brand name:Takenate D-110N, product of Takeda Chemical Ind. Ltd., ethyl acetatecontent: 25%). The solution was emulsified with cooling in the aboveaqueous medium by use of homomixer (brand name: TK homomixer Model HV-M,product of Tokushu Kika Kogyo KK) to prepare an emulsion containingparticles 2 μm in average size.

To the emulsion was added 50 parts of water and the mixture was reactedat 60° C. for 3 hours with stirring to obtain a capsule dispersioncontaining a ultraviolet absorber and having a capsule wall film of apolyurethane.polyurea resin.

(2) Composition (B)

3-(N-Ethyl-N-isoamyl)amino-6-methyl-7-anilinofluoran (10 parts), 5 partsof 5% aqueous solution of methyl cellulose and 40 parts of water werepulverized to a mean particle size of 3 μm by a sand mill to obtainComposition (B).

(3)Composition (C)

4-Hydroxy-4'-isopropoxydiphenylsulfone (30 parts), 5 parts of 5% aqueoussolution of methyl cellulose and 80 parts of water were pulverized to amean particle size of 3 μm by a sand mill to obtain Composition (C).

(4) Composition (D)

1,2-Di(3-methylphenoxy)ethane (20 parts), 5 parts of 5% aqueous solutionof methyl cellulose and 55 parts of water were pulverized to a meanparticle size of 3 μm by a sand mill to obtain Composition (D).

(5) Formation of Recording Layer

A coating composition for a recording layer prepared by mixing together55 parts of the composition B, 115 parts of the composition C, 80 partsof the composition D, 80 parts of 10% aqueous solution of polyvinylalcohol and 35 parts of calcium carbonate with stirring was applied toone surface of a wood-free paper weighing 60 g/m² in an amount of 6 g/m²when dried, followed by drying to form a heat sensitive recording layer.

(6) Formation of Protective Layer

A coating composition for a protective layer was prepared by mixingtogether 220 parts of the composition A, 150 parts of 10% aqueoussolution of acetoacetyl-modified polyvinyl alcohol (brand name:Gohsefimer Z-200, product of The Nippon Synthetic Chemical Ind. Co.,Ltd.), 15 parts of kaolin (brand name: UW-90, product of EngelhardMinerals & Chemicals Corp.), 6 parts of 30% aqueous dispersion of zincstearate and 30 parts of water with stirring.

The coating compositon was applied to the above recording layer in anamount of 6 g/m² when dried, followed by drying and supercalenderingwith use of a new-cotton roll of 85° in Shore D hardness as an elasticroll at conditions of 60 kg/cm in nip pressure and 2 in nip number toobtain a heat sensitive recording paper which has protective layersurface of 1200 seconds in Bekk smoothness.

EXAMPLE 2

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition A, 2 parts of2-[2'-hydroxy-3'5'-bis(α, α-dimethylbenzyl)phenyl]benzotriazole was usedin place of 2 parts of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

EXAMPLE 3

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition A, 2 parts of2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole was used in placeof 2 parts of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

EXAMPLE 4

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition A, 2 parts of2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole wasused in place of 2 parts of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

EXAMPLE 5

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition A, 2 parts of2-(2'-hydroxy-3', 5'-di-tert-butylphenyl)-5-chlorobenzotriazole was usedin place of 2 parts of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

EXAMPLE 6

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition A, 2 parts of2-(2'-hydroxy-3',5'-di-tert-amylphenyl)benzotriazole was used in placeof 2 parts of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

EXAMPLE 7 (1) Composition (E)

A coating compositon for an undercoat layer was prepared by mixingtogether 100 parts of calcined clay (brand name: Ansilex, product ofEngelhard Minerals & Chemicals Corp., oil absorption: 110 ml/100 g), 100parts of 10% aqueous solution of polyvinyl alcohol and 200 parts ofwater.

(2) Preparation of Heat Sensitive Recording Paper

To one surface of a wood-free paper weighing 60 g/m² was applied thecomposition E in an amount of 7 g/m² when dried, followed by drying toobtain an undercoat layer. To the layer were applied the coatingcomposition for recording layer and the coating composition forprotective layer each used in Example 1 each in an amount of 6 g/m² whendried, followed by drying an supercalendering in the same manner as inExample 1 to obtain a heat sensitive recording paper which hasprotective layer surface of 2500 seconds in Bekk Smoothness.

EXAMPLE 8

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition C, 30 parts ofbis(4-hydroxyphenylthioethoxy)-methane was used in place of 30 parts of4-hydroxy-4'-isopropoxydiphenylsulfone, and in the preparation ofcomposition D, 20 parts of bis(4-hydroxyphenylthioethoxy)-methane wasused in place of 20 parts of 1,2-di(3-methylphenoxy)ethane.

EXAMPLE 9

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition D, 20 parts of2-naphthyl benzyl ether was used in place of 20 parts of1,2-di(3-methylphenoxy)ethane.

EXAMPLE 10

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the formation of protective layer, 150 partsof 10% aqueous solution of carboxyl-modified polyvinyl alcohol (brandname: KL-318, product of Kuraray Co., Ltd.) was used in place of 150parts of 10% aqueous solution of acetoacetyl-modified polyvinyl alcohol,and 15 parts of calcium carbonate was used in place of 15 parts ofkaolin.

EXAMPLE 11

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition A, the amountof the adduct of xylylenediisocyanate and trimethylolpropane (3:1)(ethyl acetate content: 25%) was changed to 8 parts from 18 parts, andthe amount of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole was changed to1.4 parts from 2 parts.

EXAMPLE 12

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of composition A, the amountof the 2-(2'-hydroxy-5'-methylphenyl)benzotriazole was changed to 0.5part from 2 parts.

EXAMPLE 13 (1) Composition (F)

To a vessel equipped with a heater was added 150 parts of 5% aqueoussolution of polyacrylic acid and the pH was adjusted to 4.5 to obtain anaqueous medium for microencapsulation.

Separately, in 100 parts of tricresyl phosphate was disolved 16 parts of2-(2'-hydroxy-5'-methylphenyl)benzotriazole to obtain a solution of acapsule core material. The solution was emulsified at 10000 rpm for 15minutes with cooling in the above aqueous medium by use of homomixer(brand name: TK homomixer Model HV-M) to prepare an emulsion.

To the emulsion was added 190 parts of 30% aqueous solution ofcommercial melamine-formaldehyde precondensate and the mixture wasreacted at 90° C. for 3 hours with stirring and then cooled to obtain acapsule dispersion containing a ultraviolet absorber and having acapsule wall film of a melamine-formaldehyde resin. The capsuledispersion contained particles 2 μm in average size.

(2) Preparation of Heat Sensitive Recording Paper

A coating composition for a protective layer was prepared by mixingtogether 55 parts of the composition F, 150 parts of 10% aqueoussolution of acetoacetyl-modified polyvinyl alcohol (brand name:Gohsefimer Z-200), 15 parts of kaolin (brand name: UW-90), 6 parts of30% aqueous dispersion of zinc stearate and 30 parts of water withstirring.

A heat sensitive recording paper was prepared in the same manner as inExample 1 with the exception of using the above coating composition forthe protective layer. The Bekk smoothness of the protective layersurface was 1200 seconds.

COMPARISON EXAMPLE 1

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of the composition A,2-(2'-hydroxy-5'-methylphenyl)benzotriazole was not used. The Bekksmoothness of the protective layer surface was 1200 seconds.

COMPARISON EXAMPLE 2 (1) Composition (G)

2-(2'-Hydroxy-5'-methylphenyl)benzotriazole (10 parts), 5 parts of 5%aqueous solution of methyl cellulose and 40 parts of water werepulverized to a mean particle size of 3 μm by a sand mill to obtainComposition (G).

(2) Preparation of Heat Sensitive Recording Paper

A heat sensitive recording paper was prepared in the same manner as inExample 1 except that, in the preparation of the protective layer, 40parts of the composition G was used in place of 220 parts of thecomposition A. The Bekk smoothness of the protective layer surface was900 seconds.

COMPARISON EXAMPLE 3

A heat sensitive recording paper was prepared in the same manner as inComparison Example 2 except that, in the preparation of the compositionG, 10 parts of 2-(2'-hydroxy-3',5'-di-tert-amylphenyl)benzotriazole wasused in place of 10 parts of2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

The 16 kinds of heat sensitive recording papers thus obtained weretested for the following properties. Table 1 shows the results.

1. Color forming property

Each recording paper was caused to produce color images thereon using atester for heat sensitive recording materials (brand name: TH-PMD,product of Ohkura Denki Co., Ltd.) at an applied-voltage of 24 V with apulse width of 2 ms. The density of images recorded was measured by aMacbeth densitometer (Model RD-914, product of Macbeth Corp.) in avisual mode.

2. Light resistance

The heat sensitive recording paper tested by the foregoing procedure 1was exposed directly to sunlight for 2 days, and the density of thebackground area was thereafter measured by the Macbeth densitometer(using a blue filter).

3. Plasticizer resistance

The heat sensitive recording paper having the images recorded thereon bythe procedure 1 was tested for plasticizer resistance by winding a wrapfilm (brand name: KMA-W, product of Mitsui Toatsu Chemicals Inc.) arounda polycarbonate pipe (40 mm in diameter) in three lapping layers,placing the recording paper over the lapping layers, further winding thewrap film around the recording paper in three lapping layers, allowingthe resulting winding to stand at 20° C. for 24 hours and thereaftermeasuring the color density of the recorded images by the Macbethdensitometer (in a visual mode).

4. Resistance to high temperature and high humidity

The heat sensitive recording paper having the images recorded thereon bythe procedure 1 was allowed to stand in an atmosphere of 50° C. and RH95% for 24 hours, and the color density of the recorded images and thedensity of the background area were thereafter measured by the Macbethdensitometer (in a visual mode) for the evaluation of resistance to hightemperature and high humidity.

5. Solvent resistance

The surface of the heat sensitive recording paper before recording waswiped with gauze wet with ethanol, and the density of resulting fog wasmeasured by the Macbeth densitometer (in a visual mode) for theevaluation of solvent resistance.

                                      TABLE 1                                     __________________________________________________________________________    amount of    color           high-temp. and high-                             UV absorber  forming                                                                            light                                                                              plasticizer                                                                         humidity resistance                                                                        solvent                             (g/m.sup.2)  ability                                                                            resistance                                                                         resistance                                                                          record image                                                                         background                                                                          resistance                          __________________________________________________________________________    Ex. 1 0.21   1.26 0.12 1.21  1.24   0.13  0.08                                Ex. 2 0.21   1.25 0.17 1.20  1.24   0.12  0.07                                Ex. 3 0.21   1.25 0.11 1.19  1.22   0.15  0.09                                Ex. 4 0.21   1.24 0.13 1.20  1.21   0.16  0.11                                Ex. 5 0.21   1.25 0.17 1.22  1.23   0.16  0.09                                Ex. 6 0.21   1.24 0.16 1.18  1.23   0.15  0.06                                Ex. 7 0.21   1.27 0.09 1.25  1.26   0.10  0.09                                Ex. 8 0.21   1.23 0.15 1.16  1.20   0.18  0.12                                Ex. 9 0.21   1.26 0.13 1.22  1.23   0.19  0.08                                Ex. 10                                                                              0.21   1.25 0.11 1.14  1.22   0.14  0.08                                Ex. 11                                                                              0.21   1.26 0.12 1.10  1.09   0.25  0.08                                Ex. 12                                                                              0.05   1.26 0.18 1.21  1.21   0.13  0.09                                Ex. 13                                                                              0.22   1.25 0.12 1.21  1.24   0.13  0.08                                Com. Ex. 1                                                                          --     1.25 0.40 1.21  1.01   0.14  0.08                                Com. Ex. 2                                                                          1.11   1.21 0.28 0.33  0.89   0.35  0.60                                Com. Ex. 3                                                                          1.11   1.20 0.34 0.24  0.80   0.47  0.81                                __________________________________________________________________________

EXAMPLE 14 (1) Composition (H)

To a vessel equipped with a heater was added 60 parts of 8% aqueoussolution of polyvinyl alcohol (brand name: PVA-217, product of KurarayCo., Ltd.) to obtain an aqueous medium for microencapsulation.

Separately, 3 parts of ethyl acetate, 12 parts of2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole and 18 parts ofan adduct of xylylene diisocyanate and trimethylolpropane (3:1) (brandname: Takenate D-110N, product of Takeda Chemical Ind. Ltd., ethylacetate content: 25%) were mixed to obtain a solution of a capsule corematerial and capsule wall-forming material. The solution was emulsifiedwith cooling in the above aqueous medium by use of homomixer (brandname: TK homomixer Model HV-M, product of Tokushu Kika Kogyo KK) toprepare an emulsion containing particles 2 μm in average size.

To the emulsion was added 50 parts of water and the mixture was reactedat 60° C. for 3 hours with stirring to obtain a capsule dispersioncontaining a ultraviolet absorber and having a capsule wall film of apolyurethane.polyurea resin.

(2) Composition (I)

3-Di(n-butyl)amino-6-methyl-7-anilinofluoran (10 parts), 5 parts of 5%aqueous solution of methyl cellulose and 40 parts of water werepulverized to a mean particle size of 3 μm by a sand mill to obtainComposition (I).

(3) Composition (J)

4-Hydroxy-4'-isopropoxydiphenylsulfone (30 parts), 5 parts of 5% aqueoussolution of methyl cellulose and 80 parts of water were pulverized to amean particle size of 3 μm by a sand mill to obtain Composition (J).

(4) Composition (K)

1,2-Di(3-methylphenoxy)ethane (20 parts), 5 parts of 5% aqueous solutionof methyl cellulose and 55 parts of water were pulverized to a meanparticle size of 3 μm by a sand mill to obtain Composition (K).

(5) Formation of Recording Layer

A coating composition for a recording layer prepared by mixing together55 parts of the composition I, 115 parts of the composition J, 80 partsof the composition K, 80 parts of 10% aqueous solution of polyvinylalcohol and 35 parts of calcium carbonate with stirring was applied toone surface of a wood-free paper weighing 60 g/m² in an amount of 6 g/m²when dried, followed by drying to form a heat sensitive recording layer.

(6) Formation Of Protective Layer

A coating composition for a protective layer was prepared by mixingtogether 220 parts of the composition H, 150 parts of 10% aqueoussolution of acetoacetyl-modified polyvinyl alcohol (brand name:Gohsefimer Z-200, product of The Nippon Synthetic Chemical Ind. Co.,Ltd), 15 parts of kaolin (brand name: UW-90, product of EngelhardMinerals & Chemicals Corp.), 6 parts of 30% aqueous dispersion of zincstearate and 30 parts of water with stirring.

The coating composition was applied to the above recording layer in anamount of 6 g/m² when dried, followed by drying and supercalenderingwith use of a new-cotton roll of 85° in Shore D hardness as an elasticroll at conditions of 60 kg/cm in nip pressure and 2 in nip number toobtain a heat sensitive recording paper which has protective layersurface of 1200 seconds in Bekk smoothness.

EXAMPLE 15

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition H, 12 parts of2-[2'-hydroxy-4'-(2"-ethylhexyl)oxyphenyl]benzotriazole was used inplace of 12 parts of2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)-benzotriazole.

EXAMPLE 16

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition H, 12 parts ofoctyl5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzene-propionatewas used in place of 12 parts of2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole.

EXAMPLE 17

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition H, 3 parts of1-phenyl-1-methyl-1-xylylmethane was used in place of 3 parts of ethylacetate, 6 parts of2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole and 6 parts of acondensate of methyl3-[3-tert-butyl-5-(2H-benzotriazole-2-yl)-4-hydroxyphenyl]propionate andpolyethylene glycol (molecular weight: about 300) were used in place of12 parts of 2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole.

EXAMPLE 18 (1) Composition (L)

A coating composition for an undercoat layer was prepared by mixingtogether 100 parts of calcined clay (brand name: Ansilex, product ofEngelhard Minerals & Chemicals Corp., oil absorption: 110 ml/100 g), 100parts of 10% aqueous solution of polyvinyl alcohol and 200 parts ofwater.

(2) Preparation of heat sensitive recording paper

To one surface of a wood-free paper weighing 60 g/m² was applied thecomposition L in an amount of 7 g/m² when dried, followed by drying toobtain an undercoat layer. To the layer were applied the coatingcomposition for recording layer and the coating composition forprotective layer each used in Example 14 each in an amount of 6 g/m²when dried, followed by drying and supercalendering in the same manneras in Example 14 to obtain a heat sensitive recording paper which hasprotective layer surface of 2500 seconds in Bekk Smoothness.

EXAMPLE 19

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition J, 30 parts ofbis(4-hydroxyphenylthioethoxy)-methane was used in place of 30 parts of4-hydroxy-4'-isopropoxydiphenylsulfone, and in the preparation ofcomposition K, 20 parts of bis(4-hydroxyphenylthioethoxy)-methane wasused in place of 20 parts of 1,2-di(3-methylphenoxy)ethane.

EXAMPLE 20

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition K, 20 parts of2-naphthyl benzyl ether was used in place of 20 parts of1,2-di(3-methylphenoxy)ethane.

EXAMPLE 21

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the formation of protective layer, 150 partsof 10% aqueous solution of carboxyl-modified polyvinyl alcohol (brandname: KL-318, product of Kuraray Co., Ltd.) was used in place of 150parts of 10% aqueous solution of acetoacetyl-modified polyvinyl alcohol,and 15 parts of calcium carbonate was used in place of 15 parts ofkaolin.

EXAMPLE 22

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition H, the amountof the adduct of xylylene diisocyanate and trimethylolpropane (3:1)(ethyl acetate content: 25wt. %) was changed to 8 parts from 18 parts.

EXAMPLE 23

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition H, the amountof the 2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole waschanged to 3 parts from 12 parts, and 12 parts of diisopropylnaphthalenewas used in place of 3 parts of ethyl acetate.

COMPARISON EXAMPLE 4

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of composition H, 12 parts ofdiisopropylnaphthalene was used in place of 12 parts of2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole. The Bekksmoothness of the protective layer surface was 1200 seconds.

COMPARISON EXAMPLE 5 (1) Composition (M)

2-(2'-Hydroxy-5'-methylphenyl)benzotriazole (10 parts), 5 parts of 5%aqueous solution of methyl cellulose and 40 parts of water werepulverized to a mean particle size of 3 μm by a sand mill to obtainComposition (M).

(2) Preparation of heat sensitive recording paper

A heat sensitive recording paper was prepared in the same manner as inExample 14 except that, in the preparation of the protective layer, 45parts of the composition M was used in place of 220 parts of thecomposition H. The Bekk smoothness of the protective layer surface was900 seconds.

COMPARISION EXAMPLE 6

A heat sensitive recording paper was prepared in the same manner as inComparison Example 5 except that, in the preparation of the compositionM, 10 parts of 2-(2'-hydroxy-3', 5'-di-tert-amylphenyl)benzotriazole wasused in place of 10 parts of2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

The 13 kinds of heat sensitive recording papers thus obtained weretested for the following properties. Table 2 shows the results.

1. Color forming property

Each recording paper was caused to produce color images thereon using atester for heat sensitive recording materials (brand name: TH-PMD,product of Ohkura Denki Co., Ltd.) at an applied-voltage of 24 V with apulse width of 2 ms. The density of images recorded was measured by aMacbeth densitometer (Model RD-914, product of Macbeth Corp.) in avisual mode.

2. Light resistance

The heat sensitive recording paper tested by the foregoing procedure 1was exposed directly to sunlight for 7 days, and the density of thebackground area was thereafter measured by the Macbeth densitometer(using a blue filter).

3. Plasticizer resistance

The heat sensitive recording paper having the images recorded thereon bythe procedure 1 was tested for plasticizer resistance by winding a wrapfilm (brand name: KMA-W, product of Mitsui Toatsu Chemicals Inc.) arounda polycarbonate pipe (40 mm in diameter) in three lapping layers,placing the recording paper over the lapping layers, further winding thewrap film around the recording paper in three lapping layers, allowingthe resulting winding to stand at 20° C. for 24 hours and thereaftermeasuring the color density of the recorded images for the evaluation ofplasticizer resistance.

4. Resistance to high temperature and high humidity

The heat sensitive recording paper having the images recorded thereon bythe procedure 1 was allowed to stand in an atmosphere of 50° C. and RH75% for 24 hours, and the color density of the recorded images and thedensity of the background area were thereafter measured for theevaluation of resistance to high temperature and high humidity.

5. Solvent resistance

The surface of the heat sensitive recording paper before recording waswiped with gauze wet with ethanol, and the density of resulting fog wasmeasured for the evaluation of solvent resistance.

                                      TABLE 2                                     __________________________________________________________________________           UV absorber                                                                          color           high-temp. and high-                                   in protective                                                                        forming                                                                            light                                                                              plasticizer                                                                         humidity resistance                                                                        solvent                                   layer (g/m.sup.2)                                                                    ability                                                                            resistance                                                                         resistance                                                                          record image                                                                         background                                                                          resistance                         __________________________________________________________________________    Ex. 14 1.2    1.26 0.10 1.18  1.20   0.12  0.10                               Ex. 15 1.2    1.25 0.10 1.19  1.20   0.12  0.10                               Ex. 16 1.2    1.25 0.10 1.15  1.20   0.12  0.10                               Ex. 17 1.2    1.24 0.10 1.19  1.20   0.12  0.10                               Ex. 18 1.2    1.29 0.09 1.23  1.23   0.10  0.08                               Ex. 19 1.2    1.24 0.14 1.17  1.18   0.13  0.10                               Ex. 20 1.2    1.27 0.13 1.20  1.22   0.13  0.10                               Ex. 21 1.2    1.25 0.10 1.17  1.19   0.12  0.11                               Ex. 22 1.4    1.24 0.10 1.09  1.02   0.16  0.11                               Ex. 23 0.2    1.26 0.20 1.18  1.20   0.12  0.10                               Com. Ex. 4                                                                           --     1.26 0.60 1.17  1.20   0.13  0.10                               Com. Ex. 5                                                                           1.2    1.20 0.50 0.35  0.82   0.31  0.62                               Com. Ex. 6                                                                           1.2    1.17 0.54 0.22  0.81   0.45  0.78                               __________________________________________________________________________

EXAMPLE 24 (1) Composition (N)

To a vessel equipped with a heater was added 220 parts of 12% aqueoussolution of a partially saponified acetoacetyl-modified polyvinylalcohol (brand name: Gohsefimer Z-210, product of The Nippon SyntheticChemical Ind. Co., Ltd.) to obtain an aqueous medium formicroencapsulation.

Separately, 77 parts of2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole and 33 parts ofan isocyanate compound (brand name: Takenate D-170HN, product of TakedaChemical Ind. Ltd.) were mixed with heating at 40° C. to obtain asolution. Takenate D-170HN is isocyanate compound having a viscosity of600 cps as determined by a Brookfield type viscometer at 25° C. andcontaining as a main component isocyanurate type trimer of hexamethylenediisocyanate. The solution was emulsified with cooling in the aboveaqueous medium by use of homomixer (brand name: TK homomixer Model HV-M,product of Tokushu Kika Kogyo KK) to prepare an emulsion containingparticles 2 μm in average size.

To the emulsion was added 175 parts of water and the mixture was reactedat 90° C. for 5 hours with stirring to obtain a capsule dispersioncontaining a ultraviolet absorber and having a capsule wall film of apolyurethane.polyurea resin.

(2) Formation of Recording Layer

A heat sensitive recording paper was prepared in the same manner as inExample 14 with use of the compositions I, J and K prepared in Example14.

(3) Formation of Protective Layer

A heat sensitive recording paper which has protective layer was preparedin the same manner as in Example 14 except that, in the preparation ofthe protective layer, 220 parts of the composition N was used in placeof 220 parts of the composition H. The Bekk smoothness of the protectivelayer surface was 1200 seconds.

EXAMPLE 25

A heat sensitive recording paper was prepared in the same manner as inExample 24 except that, in the preparation of the composition N, 33parts of an isocyanate compound having Brookfield viscosity of 800 cps(brand name: Takenate D-175HN, product of Takeda Chemical Ind. Ltd.) wasused in place of 33 parts of the isocyanate compound having Brookfieldviscosity of 600 cps. Takenate D-175HN is isocyanate compound containingas a main component isocyanurate type trimer of hexamethylenediisocyanate.

EXAMPLE 26

A heat sensitive recording paper was prepared in the same manner as inExample 24 except that, in the preparation of the composition N, 33parts of an isocyanate compound having Brookfield viscosity of 260 cps(brand name: Takenate D-177N, product of Takeda Chemical Ind. Ltd.) wasused in place of 33 parts of the isocyanate compound having Brookfieldviscosity of 600 cps. Takenate D-177N is isocyanate compound containingas a main component isocyanurate type trimer of hexamethylenediisocyanate.

EXAMPLE 27

A heat sensitive recording paper was prepared in the same manner as inExample 24 except that, in the preparation of the composition N, 33parts of an isocyanate compound having Brookfield viscosity of 240 cps(brand name: Desmodur TPLS 2550, product of Sumitomo Bayer Urethane Co.,Ltd.) was used in place of 33 parts of the isocyanate compound havingBrookfield viscosity of 600 cps. Desmodur TPLS 2550 is isocyanatecompound containing as a main component isocyanurate type trimer ofhexamethylene diisocyanate.

EXAMPLE 28

A heat sensitive recording paper was prepared in the same manner as inExample 24 except that, in the preparation of the composition J, 30parts of 4,4'-isopropylidenediphenol was used in place of 30 parts of4-hydroxy-4'-isopropoxydiphenylsulfone.

EXAMPLE 29

A heat sensitive recording paper was prepared in the same manner as inExample 24 except that, in the preparation of the composition J, 30parts of 1,1-bis(4-hydroxyphenyl)-cyclohexane was used in place of 30parts of 4-hydroxy-4'-isopropoxydiphenylsulfone.

EXAMPLE 30

A heat sensitive recording paper was prepared in the same manner as inExample 24 except that, in the preparation of the composition I, 10parts of 3-di(n-butyl)amino-7-(o-chlorophenylamino)fluoran was used inplace of 10 parts of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, andin the preparation of the composition J, 30 parts of1,1-bis(4-hydroxyphenyl)cyclohexane was used in place of 30 parts of4-hydroxy-4'-isopropoxydiphenylsulfone.

EXAMPLE 31

A heat sensitive recording paper was prepared in the same manner as inExample 24 except that, in the preparation of the composition I, 10parts of 3-diethylamino-7-(m-trifluoromethylphenylamino)fluoran was usedin place of 10 parts of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran.

EXAMPLE 32 (1) Composition (O)

2,2'-Methylenebis(4-methyl-6-tert-butylphenol) (10 parts), 5 parts of 5%aqueous solution of methyl cellulose and 40 parts of water werepulverized to a mean particle size of 3 μm by a sand mill to obtainComposition (O).

(2) Formation of Recording Layer

A heat sensitive recording paper was prepared in the same manner as inExample 24 in the formation of the recording layer, with the exceptionof using a coating composition in which 30 parts of the abovecomposition (O) was further added.

(3) Formation of Protective Layer

A heat sensitive recording paper was prepared in the same manner as inExample 24 in the formation of the protective layer, with the exceptionof using the above heat sensitive recording paper.

The 9 kinds of heat sensitive recording papers thus obtained were testedfor the following properties. Table 3 shows the results.

1. Color forming property

Each recording paper was caused to produce color images thereon using atester for heat sensitive recording materials (brand name: TH-PMD,product of Ohkura Denki Co., Ltd.) at an energy of 0.4 mJ/dot. Thedensity of images recorded was measured by a Macbeth densitometer (ModelRD-914, product of Macbeth Corp.) in a visual mode.

2. Light resistance

The heat sensitive recording paper tested by the foregoing procedure 1was exposed directly to sunlight for 10 days, and each density of thebackground area before and after exposure was measured by the Macbethdensitometer (using a blue filter).

3. Plasticizer resistance

The heat sensitive recording paper having the images recorded thereon bythe procedure 1 was tested for plasticizer resistance by winding a wrapfilm (brand name: KMA-W, product of Mitsui Toatsu Chemicals Inc.) arounda polycarbonate pipe (40 mm in diameter) in three lapping layers,placing the recording paper over the lapping layers, further winding thewrap film around the recording paper in three lapping layers, allowingthe resulting winding to stand at 40° C. for 24 hours and thereaftermeasuring the color density of the recorded images by the Macbethdensitometer (in a visual mode) for the evaluation of plasticizerresistance.

4. Resistance to high temperature and high humidity

The heat sensitive recording paper having the images recorded thereon bythe procedure 1 was allowed to stand in an atmosphere of 50° C. and RH75% for 24 hours, and the color density of the recorded images and thedensity of the background area were thereafter measured by the Macbethdensitometer (in a visual mode) for the evaluation of resistance to hightemperature and high humidity.

5. Solvent resistance

The surface of the heat sensitive recording paper before recording waswiped with gauze wet with ethanol, and the density of resulting fog wasmeasured by the Macbeth densitometer (in a visual mode) for theevaluation of solvent resistance.

                                      TABLE 3                                     __________________________________________________________________________    color    light resistance                                                                              high-temp. and high-                                 forming  before                                                                             after                                                                              plasticizer                                                                         humidity resistance                                                                        solvent                                 ability  exposure                                                                           exposure                                                                           resistance                                                                          record image                                                                         background                                                                          resistance                              __________________________________________________________________________    Ex. 24                                                                            1.30 0.07 0.10 1.25  1.23   0.12  0.10                                    Ex. 25                                                                            1.29 0.07 0.10 1.25  1.24   0.12  0.10                                    Ex. 26                                                                            1.37 0.07 0.10 1.23  1.15   0.15  0.17                                    Ex. 27                                                                            1.37 0.07 0.09 1.23  1.17   0.14  0.15                                    Ex. 28                                                                            1.34 0.08 0.14 1.17  1.20   0.13  0.14                                    Ex. 29                                                                            1.27 0.07 0.09 1.19  1.18   0.12  0.11                                    Ex. 30                                                                            1.24 0.05 0.06 1.12  1.05   0.08  0.08                                    Ex. 31                                                                            1.30 0.06 0.07 1.25  1.20   0.10  0.08                                    Ex. 32                                                                            1.25 0.10 0.13 1.22  1.20   0.16  0.10                                    __________________________________________________________________________

EXAMPLE 33

A coating composition was prepared by mixing together 220 parts of thecomposition A obtained in Example 1, 150 parts of 10% aqueous solutionof acetoacetyl-modified polyvinyl alcohol (brand name: Gohsefimer Z-200,product of The Nippon Synthetic Chemical Ind. Co., Ltd.) and 0.3 part of40% aqueous solution of glyoxal. The coating composition was applied toa clay-coated paper having previously yellow ink printed thereon by anoff-set printing in an amount of 3 g/m² by dry weight, followed bydrying to obtain a test paper.

EXAMPLE 34

A test paper was prepared in the same manner as in Example 33 exceptthat 55 parts of the composition F prepared in Example 13 was used inplace of 220 parts of the composition A.

EXAMPLE 35

A test paper was prepared in the same manner as in Example 33 exceptthat 220 parts of the composition H prepared in Example 14 was used inplace of 220 parts of the composition A.

EXAMPLE 36

A test paper was prepared in the same manner as in Example 33 exceptthat 220 parts of the composition N prepared in Example 24 was used inplace of 220 parts of the composition A.

COMPARISON EXAMPLE 7

A test paper was prepared by printing yellow ink on a paper by anoff-set printing. This paper was not coated with microcapsulescontaining an ultraviolet absorber.

COMPARISON EXAMPLE 8 (1) Composition (P)

A quantity of 60 parts of 8% aqueous solution of acetoacetyl-modifiedpolyvinyl alcohol (brand name: Gohsefimer Z-200), 32 parts ofultrafinely divided zinc oxide (particle size: 0.005˜0.01 μm) and 50parts of water were mixed together to obtain Composition (P).

(2) Preparation of Test Paper

A test paper was prepared in the same manner as in Example 33 exceptthat 220 parts of the composition P was used in place of 220 parts ofthe composition A.

COMPARISON EXAMPLE 9

A test paper was prepared in the same manner as in Example 33 exceptthat 70 parts of the composition G prepared in Comparison Example 2 wasused in place of 220 parts of the composition A.

The 7 kinds of test papers thus obtained were tested for optical densityof the printed yellow images by a Macbeth densitometer (Model RD-914,product of Macbeth Corp.) using a blue filter. The above test paper wasexposed directly to sunlight for 10 days, and optical density of theprinted yellow images was again measured in the same manner as above forthe evaluation of light resistance. Table 4 shows the results.

                  TABLE 4                                                         ______________________________________                                                   optical density of printed image                                              before exposure                                                                         after exposure                                                      to light  to light                                                 ______________________________________                                        Ex. 33       0.70        0.60                                                 Ex. 34       0.70        0.59                                                 Ex. 35       0.70        0.66                                                 Ex. 36       0.70        0.68                                                 Com. Ex. 7   0.75        0.30                                                 Com. Ex. 8   0.49        0.41                                                 Com. Ex. 9   0.55        0.41                                                 ______________________________________                                    

As apparent from Tables 1 to 4, the present microcapsules having anultraviolet absorber enclosed therein absorb ultraviolet ray efficientlyand are excellent in preventing deterioration of a substrate againstlight. Further, a heat sensitive recording material using suchmicrocapsules is extremely low in color change of the background areaand excellent in color forming ability and record image preservability.

We claim:
 1. A heat sensitive recording material comprising a substrate,a recording layer formed over the substrate and containing a colorlessor light-colored basic dye and a color acceptor, and a protective layerformed over the recording layer, the recording material beingcharacterized in that microcapsules having an ultraviolet absorberenclosed therein and having substantially no color forming ability areincorporated in the protective layer.
 2. A heat sensitive recordingmaterial as defined in claim 1 wherein the microcapsules have meanparticle size of 0.1 to 3 μm.
 3. A heat sensitive recording material asdefined in claim 1 wherein the microcapsules have a capsule wall film ofa synthetic resin.
 4. A heat sensitive recording material as defined inclaim 3 wherein the synthetic resin is polyurea resin orpolyurethane.polyurea resin.
 5. A heat sensitive recording material asdefined in claim 4 wherein the polyurethane.polyurea resin is one whichis prepared by use of an adduct of xylylene diisocyanate andtrimethylolpropane.
 6. A heat sensitive recording material as defined inclaim 4 wherein the polyurea resin or polyurethane.polyurea resin is onewhich is prepared by use of hexamethylene diisocyanate derivative havinga viscosity of up to 1000 cps as determined by a Brookfield typeviscometer at 25° C.
 7. A heat sensitive recording material as definedin claim 6 wherein the hexamethylene diisocyanate derivative is anisocyanate compound containing an isocyanurate trimer of hexamethylenediisocyanate.
 8. A heat sensitive recording material as defined in claim3 wherein the synthetic resin is aminoaldehyde resin.
 9. A heatsensitive recording material as defined in claim 3 wherein themicrocapsules are those prepared by use of, as an emulsifier, at leastone modified polyvinyl alcohol selected from the group consisting ofacetoacetyl-modified polyvinyl alcohol, carboxyl-modified polyvinylalcohol and silicon-modified polyvinyl alcohol.
 10. A heat sensitiverecording material as defined in claim 1 wherein the ultravioletabsorber is a benzotriazole derivative which is liquid at ordinarytemperature.
 11. A heat sensitive recording material as defined in claim10 wherein the benzotriazole derivative is at least one selected fromthe group consisting of2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole,2-[2'-hydroxy-4'-(2"-ethylhexyl)oxyphenyl]benzotriazole, octyl5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzene-propionate,and condensate of methyl3-[3-tert-butyl-5-(2H-benzotriazole-2-yl)-4-hydroxyphenyl]propionate andpolyethylene glycol (molecular weight: about 300).